1. Field of the Invention
The invention relates to DC voltage converters which convert a direct current power input at one voltage level to a direct current power output at a different voltage level, and more particularly to the application of staggered mode switching to single-ended, pulse-width-modulated, DC to DC converters, thereby, combining the benefits of a transformer coupled supply with the benefits of staggered mode switching.
2. Description of Prior Art
The use of staggered mode switching to reduce the RMS value of the AC currents and voltages inherent in switching regulators and the resulting improvement in output ripple voltage and filter size and cost have been disclosed in the art.
Staggered mode switching has been applied in the prior art with one or more of the following disadvantages:
1. No DC isolation between the input and output. PA1 2. A restriction in the nominal voltage transformation ratio in order to receive the maximum benefit obtainable from staggered mode operation. PA1 3. Current steps in the output filter capacitor require the capacitor to have a very low impedance over a wide frequency range to control the ripple and switching spikes in the output voltage. PA1 4. A circuit topology that does not reduce the RMS AC currents and voltages to the degree that it is achievable through the staggered mode operation of two single ended converters. PA1 5. More than two power transistors are required. PA1 6. More than one voltage control is required. PA1 7. More than one overload protection circuit is required. PA1 8. Accurate balance of the power and stress levels between the converters or power channels cannot be achieved. PA1 9. Power transformer windings are operated at a lower duty cycle thus adding to their size and weight. PA1 10. High pulse current in the output capacitor over most of the input voltage range greatly increases the size, weight and cost of the output filter capacitor. PA1 11. The converter must be protected against simultaneous conduction of the power switching transistors. PA1 12. A blocking capacitor or some other means to control the dc flux imbalance in the power transformer is required since the transformer cannot be allowed to saturate at either end of the cycle.
Accordingly, it is an object of the present invention to provide a DC to DC converter that has a combination of characteristics which will enable it to overcome the aforementioned disadvantages and which make it particularly well suited to the requirements of avionic equipment. Such avionic equipment requirements which this novel invention can meet include small size and weight, the utilization of low profile components so as to be compatible with standard hardware module packaging, a two:one input voltage range, high efficiency, operability over a wide component temperature range (minus 55.degree. C. to .+-.125.degree. C.) and high reliability.